Various types of circuitry have been developed to provide a drive signal to rotate a mirror of a galvanometer to a required position or the mirror of a light scanning apparatus in a predetermined pattern. One type of circuit is shown in U.S. Pat. No. 4,286,212 in which a variable control signal generator is coupled to input terminals of the galvanometer through a current amplifier, the angle of the deflection of the mirror being dependent on the value of the control signal from the generator. However, due to the fact that the galvanometer exhibits hysteresis, the deflection angle also depends to a certain extent on the value of the previously applied control signal. This hysteresis makes it difficult to achieve repeatable random access of any particular discrete angular position of the mirror with great accuracy. In order to counteract this problem, U.S. Pat. No. 4,286,212 provides a system to ensure that the control signal applied to the galvanometer is always returned to a predetermined reference level for a given period of time before it is changed to a new value. Therefore, each new angle to which it is desired to accurately rotate the mirror is referenced from substantially the same reference position to eliminate the effect of hysteresis on the movement of the mirror.
U.S. Pat. No. 4,329,011 discloses a laser printer in which a laser beam is reflected by a galvanometer mirror onto a recording medium. The drive signals are supplied to the drive for the mirror from a drive signal generator through a drive amplifier. The drive signal generator produces a number of different voltages which are applied by electronic switches at particular times in a scanning cycle to the drive amplifier. These then provide a drive signal which will rotate the galvanometer, taking into account the resonant frequency of the galvanometer, in a manner which will scan the laser beam across the recording medium at a high and constant speed.
U.S. Pat. No. 4,368,489 describes a system for scanning frames of motion-picture films including a tilting-mirror scanning mechanism which deflects the projected image of a frame across a row of photodiodes, in a direction perpendicular to the row, periodically at a frequency corresponding to the vertical scanning frequency of a standard television picture. The tilting-mirror mechanism employed is provided with oil-damped action with the damping oil being maintained at a stabilized temperature by a heating device in order to provide a stabilized damping action. The movement of the tilting-mirror in this device is temperature dependent due to temperature effects on the damping action of the oil. Other types of scanning devices also have mechanism whose movements with applied drive signals can vary with changes in temperature. This U.S. Pat. No. 4,368,489 illustrates the type of control signal, as well as circuitry for generating the signal, which will move the tilting-mirror scanning mechanism with the required scanning sweep.
The above-mentioned references serve to illustrate various types of mechanism employing rotatable or scanning mirrors and indicates that the control signals needed for driving these mirrors vary considerably in the shape of their waveform depending on the type of mechanism and required motion. U.S. Pat. Nos. 4,329,011 and 4,368,489 use pre-encoded waveforms to generate the desired scanning motion of their mirrors. These pre-encoded waveforms are fixed for any one particular type of galvanometer. However, it is impossible to perfectly predict the response of any one type of device to a pre-defined excitation. The motion, as a result can fluctuate considerably from one device to another for a particular drive signal waveform and these devices, along with their drive signal waveform, must then be manually adjusted. The characteristics of galvanometers or scanning mirror devices can also change over a period of time due to temperature changes, aging, drifting etc.
U.S. Pat. Nos. 4,329,011 and 4,368,489 provide an angular displacement to their mirrors which is characterized by a, more or less, sawtooth shape, i.e. a linear rotational motion of the mirror in one direction followed by a faster return motion. However, other types of rotational motion of galvanometers mirrors exist such as sinusoidal rate of movement. Due to the inertia of the moving mirror and friction in the system, the voltage waveform required to move the mirror in a particular manner will not correspond to the waveform of the actual angular displacement of the mirror. This is clearly illustrated in U.S. Pat. No. 4,648,685 which show diagrams of the waveforms of voltages applied to drive a galvanometer mirror and the responsive angular displacements of the galvanometer mirror in the same figures.
U.S. Pat. No. 4,350,988 shows a system for scanning a laser beam across a recording medium by moving a mirror which is driven by a motor so that the angular displacement of the mirror has a sinusoidal form. A photodetector with a mask having two parallel slits is positioned so that the laser beam is reflected towards the detector when it is near one end of its scan. The detector then provides two pulses and from these, by measuring the pulse interval, the scanning velocity at that position can be determined. A signal proportional to the detected velocity and a signal from a reference signal generator are applied to a comparison circuit which provides an output signal (error signal) to adjust the speed of the motor and control the velocity of the beam as it scans the recording medium.